1. Field of the Invention
This invention relates to a yarn useful for the production of protective fabrics and garments, and fabrics and garments that possess not only arc and flame protective properties, but also improved performance when exposed to flash fires.
2. Description of Related Art
When protecting workers from potential flash fires with protective apparel, the time of exposure to actual flame is an important consideration. Generally the term “flash” fire is used because the exposure to flame is of very short duration, on the order of seconds. Further, while the difference in a single second seems small, when exposed to fire, an additional second of exposure to a flame can mean a tremendous difference in the burn injury.
The performance of a material in a flash fire can be measured using an instrumented mannequin using the test protocol of ASTM F1930. The mannequin is clothed in the material to be measured, and then exposed to flames from burners; temperature sensors distributed throughout the mannequin measure the local temperature experienced by the mannequin that would be the temperatures experienced by a human body if subjected to the same amount of flames. Given a standard flame intensity, the extent of the burns that would be experienced by a human, (i.e., first degree, second degree, etc.) and the percent of the body burned can be determined from the mannequin temperature data.
U.S. Pat. No. 7,348,059 to Zhu et al. discloses modacrylic/aramid fiber blends for use in arc and flame protective fabrics and garments. Such blends have on average a high content (40-70 weight percent) modacrylic fiber and lower content (10 to 40 weight percent) meta-aramid fiber having a degree of crystallinity of at least 20%, and para-aramid fiber (5 to 20 weight percent). Fabrics and garments made from such blends provide protection from electrical arcs and exposures to flash fires up to 3 seconds. United States Patent Application Publication US2005/0025963 to Zhu discloses an improved fire retardant blend, yarn, fabric and article of clothing made from a blend of 10-75 parts of at least one aramid staple fiber, 15 to 80 parts by weight of at least one modacrylic staple fiber, and 5 to 30 parts by weight of at least one aliphatic polyamide staple fiber. This blend will not provide a Category 2 arc rating for fabrics in the range of 186.5 to 237 grams per square meter (5.5 to 7 ounces per square yard) because of the high proportion of flammable aliphatic polyamide fiber in this blend. U.S. Pat. No. 7,156,883 to Lovasic et al. discloses a fiber blend, fabrics, and protective garments comprising amorphous meta-aramid fiber, crystallized meta-aramid fiber, and flame retardant cellulosic fiber, the meta-aramid fiber being 50 to 85 weight percent with one to two thirds of the meta-aramid fiber being amorphous and with two to one third of the meta-aramid fiber being crystalline. Again, fabrics made by these blends would not provide a Category 2 arc rating for fabrics in the range of 186.5 to 237 grams per square meter (5.5 to 7 ounces per square yard).
The minimum performance required for flash fire protective apparel, per the NFPA 2112 standard, is less than 50% body burn from a 3 second flame exposure. Since flash fire is a very real threat to workers in some industries, and it is not possible to fully anticipate how long the individual will be engulfed in flames, any improvement in the flash fire performance of protective apparel fabrics and garments has the potential to save lives. In particular, if the protective apparel can provide enhanced protection to fire exposure above 3 seconds, e.g. 4 seconds or more, this represents an increase in potential exposure time of as much as 33% or more. Flash fires represent one of the most extreme types of thermal threat a worker can experience; such threats are much more severe than the simple exposure to a flame.
U.S. patent application Ser. No. 12/218,215 filed Jul. 11, 2008, to Zhu relates to yarn for use in arc and flame protection, and fabrics and garments made from that yarn, the yarn consisting essentially of from (a) 50 to 80 weight percent meta-aramid fiber having a degree of crystallinity of at least 20%, (b)10 to 30 weight percent modacrylic fiber, (c) 5 to 20 weight percent para-aramid fiber, and (d) 1 to 3 weight percent antistatic fiber based on the total weight of components (a), (b), (c) and (d). The fabrics and garments have a basis weight in the range of 186.5 to 237 grams per square meter (5.5 to 7 ounces per square yard) In one embodiment, garments made from the yarn provide thermal protection such that a wearer would experience less than a 65 percent predicted body burn when exposed to a flash fire exposure of 4 seconds per ASTM F1930, while maintaining a Category 2 arc rating. The modacrylic fibers are said to include fibers having 2 to 40 weight percent antimony compounds; antimony is a known heavy metal with the potential for safe disposal considerations.
Arc and flame protection deals with the saving of human life, therefore any improvement that provides the combination of improved flash fire performance with a high level of arc protection at a low basis weight is desired. Especially desired is any improvement that also provides a potentially reduced environmental footprint.